The present invention relates generally to systems and methods for medical imaging and, particularly, to systems and methods for reducing partial scan reconstruction artifacts in computed tomography perfusion (CTP).
In a computed tomography (“CT”) system, an x-ray source projects a fan-shaped or cone-shaped beam of x-rays that is collimated to lie within an x-y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object, and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all of the detectors are acquired separately to produce a transmission profile at a particular view angle.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane, and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view,” and a “scan” of the object includes a set of views acquired at different angular orientations during one revolution of the x-ray source and detector, thereby giving rise to the term “view angle” as synonymous with “view” or “projection.” In a two dimensional (2D) scan, data is processed to reconstruct an image that corresponds to a slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers,” or “Hounsfield units,” which are used to control the brightness of a corresponding pixel on a display.
Myocardial CTP is important for evaluating the hemodynamic significance of coronary artery disease. The rapid developments in multidetector row CT, particularly its successful adoption for noninvasive cardiac imaging, raise the possibility of obtaining both anatomic and functional information from a single imaging modality. However, from a technical perspective, cardiac CT is challenging due to demands in temporal resolution. Strategies to improve temporal resolution include faster gantry rotation, dual- and multiple-source CT, and partial (or short) scan reconstructions. In myocardial CTP several consecutive partial scans are used to track the transient arrival and washout of intravascular contrast agent. Since myocardial CTP protocols are typically ECG-gated, according to a desired interval within the cardiac cycle (for example, diastolic phase), it is not possible to guarantee that the same angular data range (or view angles) is covered by each consecutive partial scan. As a consequence, artifactual CT number variations over time, called partial scan reconstruction artifacts (PSR artifacts or PSAs), arise due to minor deviations in the projection data due to physical effects, for example, such as scattering and beam hardening.
Solutions proposed to address PSAs include an invasive procedure in which the animal heart is paced in synchrony with the gantry rotation of a CT scanner, thus guaranteeing consistent angular ranges for each partial scan. This results in effective reduction or elimination of artifacts. However, the obvious disadvantage of such an approach is its invasiveness, which precludes its use in humans. More recently, two noninvasive methods to reduce PSAs have been proposed. The first uses a special data acquisition mode in which x-rays are applied to complete a full gantry rotation (full scan with 360 degrees of projection data). Then, based on the ECG signal, a specific phase of the cardiac cycle is selected and both a full and a partial scan are reconstructed. And by the use of convolution operations, a new image dataset containing the low spatial frequencies portion from a full scan (with no PSAs) and high spatial frequencies from a partial scan (free from motion artifact) is obtained. Such an approach is called the “targeted spatial frequency filter” (TSFF), and is currently implemented in the second generation dual source CT scanner (Somatom Definition Flash, Siemens Healthcare). A disadvantage of this approach is an increase in radiation dose (for example, 14%), and potential decreases in temporal resolution compared to a partial scan.
Another approach to PSAs was proposed by Stenner et al. (2010). In this approach, only partial scans from a myocardial CTP sequence are used. In this approach, the raw data of several consecutive partial scans are averaged such that a sinogram containing 360 degrees of projection data (a full scan) is created. This is referred to as the “artificial full scan sinogram.” Then, a virtual partial scan sinogram is extracted from the artificial full scan sinogram, by selecting the projection data corresponding to an angular range identical to that of the acquired partial scan sinogram. Then, three image datasets are reconstructed from each corresponding sinogram. The last operation consists in subtracting the virtual scan image from the artificial scan image, leaving as a result the ‘artifact’ image. Then, the ‘artifact image’ is substrated from the original partial scan image resulting in a partial scan image free from artifact. Advantages of this method include that it is noninvasive, there is no added radiation dose, and temporal resolution is preserved. A disadvantage is that the method requires several image reconstruction steps (3 instead of 1); hence, computational requirements are increased at least threefold. Another disadvantage is that, in practice, the superposition operations are potentially error prone because of mismatches generated by motion artifacts due to the rapid heart motion, that is, when applying this technique in vivo.
Therefore, it would be desirable to have a system and method for the reduction or control of PSAs in CTP that is substantially non-invasive, that does not increase radiation dose, that preserves temporal resolution, and that is robust to motion artifacts. Additionally, it would be desirable to have a system and method for PSAs reduction that can be readily applied to existing myocardial CTP protocols available in commercial CT scanners.